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Investment Securities (Fair Value Measurement by Level) (Details) $ in Millions | Sep. 30, 2018 USD ($) Investment | Dec. 31, 2017 USD ($) Investment | Sep. 30, 2017 USD ($) | Dec. 31, 2016 USD ($)
--- | --- | --- | --- | ---
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | $ 2,365.1 | $ 3,208.3 | $ 2,509.4 | $ 2,547.3
Level 1 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 897.4 | 890.4 | 733.6 | 279.5
Level 2 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | $ 1,178.0 | $ 2,105.4 | 1,525.2 | 2,093.8
Number of hedge funds | 2 |  |  |
Common Stock |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Number of hedge funds | Investment | 2 | 2 |  |
Debt securities issued by corporations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | $ 500.0 | $ 880.9 |  |
Mortgage-backed and asset-backed securities |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 128.5 | 694.7 |  |
Mortgage-backed and asset-backed securities | Level 2 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 128.5 | 694.7 |  |
Mortgage-backed and asset-backed securities | Level 3 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fixed maturity investments | Level 3 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 | 5.6 | 0.0
Other long-term investments | Level 3 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 138.1 | 87.2 | 90.1 | 91.4
Hedge and private equity funds included in other long-term investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Other long-term investments | 151.6 | 125.3 |  |
Hedge and private equity funds included in other long-term investments | Level 3 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 151.6 | 125.3 | 154.9 | 82.6
Fair value measured on a recurring basis |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 2,527.2 | 3,259.1 |  |
Fair value measured on a recurring basis | Level 1 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 1,198.1 | 1,041.4 |  |
Fair value measured on a recurring basis | Level 2 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 1,191.0 | 2,130.5 |  |
Fair value measured on a recurring basis | Level 3 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 138.1 | 87.2 |  |
Fair value measured on a recurring basis | U.S. Government and agency obligations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 125.4 | 296.5 |  |
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 1 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 125.4 | 296.5 |  |
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 2 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 3 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 500.0 | 880.9 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 2 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 500.0 | 880.9 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 84.2 | 185.1 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer | Level 1 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer | Level 2 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 84.2 | 185.1 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer | Level 3 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 16.9 | 108.9 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities | Level 1 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities | Level 2 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 16.9 | 108.9 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities | Level 3 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 53.9 | 94.3 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care | Level 1 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care | Level 2 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 53.9 | 94.3 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care | Level 3 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 22.4 | 127.8 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications | Level 1 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications | Level 2 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 22.4 | 127.8 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications | Level 3 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 22.2 | 95.5 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials | Level 1 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials | Level 2 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 22.2 | 95.5 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials | Level 3 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 145.9 | 114.8 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials | Level 1 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials | Level 2 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 145.9 | 114.8 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials | Level 3 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments |  | 0.0 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 60.4 | 80.5 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology | Level 1 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology | Level 2 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 60.4 | 80.5 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology | Level 3 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 39.5 | 25.9 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial | Level 1 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial | Level 2 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 39.5 | 25.9 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial | Level 3 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 54.6 | 48.1 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy | Level 1 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy | Level 2 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 54.6 | 48.1 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy | Level 3 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 128.5 | 694.7 |  |
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities | Level 1 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities | Level 2 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 128.5 | 694.7 |  |
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities | Level 3 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Municipal obligations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 278.0 | 254.9 |  |
Fair value measured on a recurring basis | Municipal obligations | Level 1 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Municipal obligations | Level 2 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 278.0 | 254.9 |  |
Fair value measured on a recurring basis | Municipal obligations | Level 3 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments |  | 2.7 |  |
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations | Level 1 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments |  | 0.0 |  |
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations | Level 2 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments |  | 2.7 |  |
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations | Level 3 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments |  | 0.0 |  |
Fair value measured on a recurring basis | Fixed maturity investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 1,031.9 | 2,129.7 |  |
Fair value measured on a recurring basis | Fixed maturity investments | Level 1 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 125.4 | 296.5 |  |
Fair value measured on a recurring basis | Fixed maturity investments | Level 2 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 906.5 | 1,833.2 |  |
Fair value measured on a recurring basis | Fixed maturity investments | Level 3 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Short-term investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments |  | 176.1 |  |
Short-term investments, at fair value | 313.7 |  |  |
Fair value measured on a recurring basis | Short-term investments | Level 1 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 300.7 | 151.0 |  |
Short-term investments, at fair value |  | 176.1 | 786.5 | 175.0
Fair value measured on a recurring basis | Short-term investments | Level 2 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 13.0 | 25.1 |  |
Fair value measured on a recurring basis | Short-term investments | Level 3 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Common equity securities | Consumer |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 9.8 | 10.7 |  |
Fair value measured on a recurring basis | Common equity securities | Consumer | Level 1 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 9.8 | 10.7 |  |
Fair value measured on a recurring basis | Common equity securities | Consumer | Level 2 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Common equity securities | Consumer | Level 3 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Common equity securities | Health Care |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 16.4 | 17.1 |  |
Fair value measured on a recurring basis | Common equity securities | Health Care | Level 1 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 16.4 | 17.1 |  |
Fair value measured on a recurring basis | Common equity securities | Health Care | Level 2 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Common equity securities | Health Care | Level 3 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Common equity securities | Communications |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 14.8 | 10.9 |  |
Fair value measured on a recurring basis | Common equity securities | Communications | Level 1 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 14.8 | 10.9 |  |
Fair value measured on a recurring basis | Common equity securities | Communications | Level 2 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Common equity securities | Communications | Level 3 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Common equity securities | Financials |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 16.7 | 16.3 |  |
Fair value measured on a recurring basis | Common equity securities | Financials | Level 1 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 16.7 | 16.3 |  |
Fair value measured on a recurring basis | Common equity securities | Financials | Level 2 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Common equity securities | Financials | Level 3 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Common equity securities | Technology |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 16.8 | 15.1 |  |
Fair value measured on a recurring basis | Common equity securities | Technology | Level 1 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 16.8 | 15.1 |  |
Fair value measured on a recurring basis | Common equity securities | Technology | Level 2 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Common equity securities | Technology | Level 3 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Common equity securities | Industrial |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 15.6 | 11.9 |  |
Fair value measured on a recurring basis | Common equity securities | Industrial | Level 1 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 15.6 | 11.9 |  |
Fair value measured on a recurring basis | Common equity securities | Industrial | Level 2 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Common equity securities | Industrial | Level 3 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Common equity securities | Energy |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 6.6 | 3.8 |  |
Fair value measured on a recurring basis | Common equity securities | Energy | Level 1 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 6.6 | 3.8 |  |
Fair value measured on a recurring basis | Common equity securities | Energy | Level 2 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Common equity securities | Energy | Level 3 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Common equity securities | Exchange traded funds |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 742.9 | 569.7 |  |
Fair value measured on a recurring basis | Common equity securities | Exchange traded funds | Level 1 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 675.3 | 508.1 |  |
Fair value measured on a recurring basis | Common equity securities | Exchange traded funds | Level 2 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 67.6 | 61.6 |  |
Fair value measured on a recurring basis | Common equity securities | Exchange traded funds | Level 3 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Common equity securities | Other |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 203.9 | 210.6 |  |
Fair value measured on a recurring basis | Common equity securities | Other | Level 1 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Common equity securities | Other | Level 2 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 203.9 | 210.6 |  |
Fair value measured on a recurring basis | Common equity securities | Other | Level 3 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Common equity securities | Common equity securities |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 1,043.5 | 866.1 |  |
Fair value measured on a recurring basis | Common equity securities | Common equity securities | Level 1 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 772.0 | 593.9 |  |
Fair value measured on a recurring basis | Common equity securities | Common equity securities | Level 2 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 271.5 | 272.2 |  |
Fair value measured on a recurring basis | Common equity securities | Common equity securities | Level 3 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Other long-term investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 138.1 | 87.2 |  |
Fair value measured on a recurring basis | Other long-term investments | Level 1 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Other long-term investments | Level 2 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Other long-term investments | Level 3 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | $ 138.1 | 87.2 |  |
Forward Contracts | Fair value measured on a recurring basis | Other long-term investments | Level 3 Inputs |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Derivative, fair value, net |  | $ (3.7) | $ (15.4) | $ (1.2)

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